1. Field of the Invention
The present invention relates to pressure transient testing of producing hydrocarbon (oil and gas) reservoirs, and more particularly to measuring inter-reservoir cross flow rates between adjacent reservoir layers in connection with such pressure transient testing.
2. Description of the Related Art
Pressure transient tests are run on most wells in newly discovered and already producing hydrocarbon reservoirs. The results of such tests become a fundamental basis of assessing any future commercial producibility of the hydrocarbon (oil and gas) reservoirs, which includes important details such as economic forecasts based on predicted production rates, reserve assessments, and plans for development of infrastructure to produce and transport the hydrocarbons to markets and consumers.
During transient tests, both the production rates of fluids at surface and the pressures at downhole conditions are measured with time. Fluid samples are also collected and analyzed later in the laboratory for determining the engineering properties. The test data is analyzed in conjunction with fluid properties to characterize the reservoirs. Such an analysis includes comparing the test data with the predicted or synthetic response of a conceptual model of the actual reservoir. It is important to utilize a realistic model of the reservoir for predicting its future commercial producibility.
Transient tests are performed on new, exploratory and development wells to assess the reservoir productivity in commercial scale. Reservoir permeability and/or mobility, formation damage parameter in terms of skin factors, reservoir pressure, reservoir size and shape, locations of geological features or boundaries are important parameters that are usually determined through such tests. To ascertain accuracy of the reservoir parameters, often individual reservoir layers are tested separately. There are often adjacent reservoir layers to an active layer where wells are drilled through and produced from.
These adjacent layers are frequently separated from the active layer through what are known as tight streaks. Tight streaks are formed of semi-permeable, non-reservoir strata whose thicknesses can vary from a few inches to few hundred feet. Hence, during production from one layer (active layer), the fluid from an adjacent layer can migrate to the producing (active) layer through the tight streaks in the reservoir. With time, the pressure differential between the active and the adjacent layer can grow due to continuous production from the active layer. In addition, the flow area between the active and the adjacent layers can grow with time for a given reservoir system. These two changing conditions with time can contribute to substantial amounts of crossflow rates. Crossflow of fluid from one layer to the other within the reservoir complicates the assessment of the commercial producibility of the active layer.
Failure to account for crossflow between adjacent layers in the reservoir may mislead regarding the source of produced fluids and may thus provide unrealistic results from transient tests, where stakes are high. Transient tests provide the characteristic parameters of the reservoir that are acquired under a dynamic condition, which resembles an actual producing condition of a well.
During pressure transient tests, reservoir permeability and/or mobility, formation damage parameter in terms of skin factors, reservoir pressure, reservoir size and shape, locations of geological features or boundaries are important parameters that are usually determined through such tests. To ascertain accuracy of the reservoir parameters, often individual reservoir layers located at different vertical depths are tested separately. The layers are usually separated by semi-permeable or impermeable, non-reservoir strata whose thicknesses can vary from a few inches to few hundred feet. However, as mentioned, there are often adjacent reservoir layers to an active layer which are separated from the active layer through what are known as semi-permeable tight streaks.
In evaluating the productive capability of a subsurface reservoir layer, a test known as a transient pressure test is conducted for the layer under investigation. Sometimes, fluid from an adjacent layer can contribute to the total production from the active layer. For maximizing the hydrocarbon recovery from reservoirs under such a production arrangement, the operator of the oil or gas field needs to know the producibility of individual reservoir layers. The flow from adjacent layers interferes with accurate layer flow measurement. Such interference can cause an overestimation of the producibility of the layer under investigation. Failure to gain this a priori knowledge may cause loss of hydrocarbons from some reservoir layers due to diversion of this fluid from one reservoir layer to another layer instead of flowing towards the wellbore during production, or even shut-in.